Today we celebrate the Feast of the Most Holy Trinity, also called Trinity Sunday. It is celebrated in the Christian churches on the Sunday following Pentecost (the 50th day after Easter). It is known that the feast was celebrated on this day from as early as the 10th century. Celebration of the feast gradually spread in the churches of northern Europe, and in 1334 Pope John XXII approved it for the entire church.

As we celebrate today the Feast of the Holy Trinity we confess, adore and honor God the Father, God the Son and God the Holy Spirit the three divine persons, yet co-eternal, co-substantial, one and the same God. This celebration is not only a credal affirmation of our belief in the Trinity of Persons and Unity of Godhead but it is it also a remembrance of our origin and destiny: the God we personally believe from who we came is the same God who calls us back home.

The mystery of the Most Holy Trinity is the central mystery of Christian faith and the life. It is the mystery of God in himself. It is therefore the source of all the other mysteries of faith, the light that enlightens them. It is the most fundamental and essential teaching in the “hierarchy of the truths of faith” (GCD 43). The whole history of salvation is identical with the history of the way and the means by which the one true God, Father, Son and Holy Spirit, reveals himself to men “and reconciles and unites with himself those who turn away from sin” (GCD 47; CCC 234).

The trinity (from “trias” of which the Latin “trinitas” is a translation was first coined by Theophilus of Antioch (circa 180)) is a mystery of faith in the strict sense, one of the mysteries that are hidden in God, which can never be known unless they are revealed by God” (Dei Filius 4: DS 3015). To be sure, God has left traces of his Trinitarian being in his work of creation and in his Revelation throughout Old Testament. But his inmost being as Holy Trinity is a mystery that is inaccessible to reason alone or even to Israel’s faith before the Incarnation of God’s Son and the sending of the Holy Spirit.

The story is told of St Augustine of Hippo, a great philosopher and theologian. He was preoccupied with the doctrine of the Blessed Trinity. He wanted so much to understand the doctrine of one God in three persons and to be able to explain it logically. One day he was walking along the sea shore and reflecting on this matter. Suddenly, he saw a little child all alone on the shore. The child made a whole in the sand, ran to the sea with a little cup, filled her cup with sea water, ran up and emptied the cup into the hole she had made in the sand. Back and forth she went to the sea, filled her cup and came and poured it into the hole. Augustine drew up and said to her, “Little child, what are you doing?” She replied, “I am trying to empty the sea into this hole.” “How do you think,” Augustine asked her, “that you can empty this immense sea into this tiny hole and with this tiny cup?” She answered back, “And you, how do you suppose that with your small head you can comprehend the immensity of God?” With that the child disappeared.

The doctrine of the inner relationship of the Father, Son and Holy Spirit in such a way that each of them is fully and equally God, yet there are not three Gods but one, cannot be fully comprehended by the human mind. It is a mystery. A mystery is a truth which we are not merely incapable of discovering apart from Divine Revelation, but which, even when revealed, remains “hidden by the veil of faith and enveloped, so to speak, by a kind of darkness” (Const., “De fide. cath.”, iv). St. Jerome says, in a well-known phrase: “The true profession of the mystery of the Trinity is to own that we do not comprehend it” (De mysterio Trinitatus recta confessio est ignoratio scientiae — “Proem ad 1. xviii in Isai.”).

Though trinity is a mystery whose whole truth is unfathomable by human reason alone we can somehow comprehend a part of it when Jesus the Son of God made man (Jn 1:14) revealed God the Father (Mt 11:27) and sent his Holy Spirit to teach us and guide us into all the truth (Jn 14:17, 26 16:13). This is our dogma of the Holy Trinity:

The Trinity is One. We do not confess three Gods, but one God in three persons, the cosubstantial Trinity” (Council of Constantinople II (553): DS 421). The divine persons do not share the one divinity among themselves but each of them is God whole and entire (cf. CCC 253).

The divine Unity is Triune. The divine persons are really distinct from one another in their relation of origin: “It is the Father who generates, the Son who is begotten, and the Holy Spirit who proceeds” Lateral Council IV (1215): DS 804; cf. CCC 254).

The divine persons are relative to one another. Because it does not divide the divine unity, the real distinction of the persons from one another resides solely in the relationship which relate them to one another…While they are called three persons in view of their relationships, we believe in one nature or substance (Council of Toledo XI (675) DS 528). “Because of that unity the Father is wholly in the Son and wholly in the Holy Spirit; the Son wholly in the Father and wholly in the Holy Spirit; the Holy Spirit is wholly in the Father and wholly in the Son” (Council of Florence (1442): DS 1331).

As we profess our faith today in the mystery of the Holy Trinity may we also pray to them to help us become a true child of God our Father, a living image of Jesus his Son and a consecrated temple of the Holy Spirit. Let us also pray as Christ did that all may be one as God the Father, God the Son and God the Spirit are one and heed Christ command after he has risen from the dead: “Go and teach all nations, baptizing them in the name of the Father, and of the Son, and of the Holy Ghost” (Matthew 28:18).